This invention relates to a method and an apparatus for making an optical fiber preform, and more particularly, to an improved method and apparatus for making an optical fiber preform, wherein a large-sized mother ingot for optical fiber is thermally drawn along a vertical direction and is melted to continuously obtain preform rods with tapered portions at opposite ends thereof, and wherein deposition of a so-called silica cloud, generated in the course of melting is effectively prevented.
An optical fiber preform is obtained by drawing a mother ingot to produce a preform having a given diameter, depending on a fiber drawing machine to be used. If the ingot has a bend, such a bend is corrected during the course of drawing. Thereafter, a dummy glass is welded to the preform at opposite ends thereof, followed by drawing with a fiber drawing machine. An optical fiber made of silica glass, which has flaws in the surface thereof becomes ver embrittled. Accordingly, if an optical fiber is flawed at the time of fiber drawing of an optical fiber preform, its strength is reduced. For the purpose of suppressing the occurrence of flaws, fire polishing has usually been effected, wherein after finishing with a given diameter, an optical fiber preform is exposed to a weak flame to remove foreign matter from the outer surfaces thereof.
As is known in the art, when a preform is heated until its surface temperature reaches about 2000xc2x0 C., part of the silica glass sublimates into SiO. This SiO combines with moisture present in the surrounding atmosphere and is converted again in to fine glass particles, followed by re-deposition onto the surface of the preform. It is also known that when silica glass is fused (i.e. melted) by means of a flame, a so-called silica cloud appears just outside the strongly heated portion. This cloud has the possibility of causing flaws in the fiber surface during fiber drawing. Thus, it is necessary that the cloud be removed prior to fiber drawing. The cloud may be removed by slowly heating the preform rod with a relatively weak flame. In this connection, however, the heating of the preform rod may become inadequate, depending on the amounts of gases used and the moving speed of a burner. This leads to great strain being left in the preform rod, with the great possibility that only a slight degree of impact applied to the preform rod will result in cracking. On the other hand, when the preform rod is heated to an extent greater than required, residual strain is reduced, but the band-shaped silica cloud reappears.
To avoid this, it is common to measure a residual strain by use of a strain gauge, and to determine fire polishing conditions in such a way that gas conditions and burner moving speed conditions, under which a residual strain is at a level involving no problem therein, and also gas conditions and burner moving speed conditions, which are determined by appearance inspection, are determined by trial and error. As a matter of course, these conditions differ depending on the diameter of the perform and the nature of the individual burner. Accordingly, determining these conditions has, in fact, required much labor and time. In addition, in view of the results of the determination of these conditions, it is required that, in order not to cause silica cloud to develop, a relatively weak flame be used so that the surface temperature of a perform is not raised, and that in order to make a small residual strain, the moving speed of a burner sufficiently below to permit heat to be satisfactorily transmitted to the inside of the preform. These conditions require much time. More particularly, fusion of a preform by conventional means allows a silica cloud to be deposited, and once again requires fire polishing at a final stage. The work of determining the final-stage fire polishing conditions further requires much time and labor.
It is therefore an object of the invention to provide a method for melting an optical fiber preform to form preform rods, while preventing deposition of a silica cloud thereon. A method and apparatus according to the present invention provides for a continuous process of manufacturing preform rods, which have tapered portions at opposite ends thereof, by drawing a large-sized mother ingot in an electric furnace in a vertical direction, and subsequently melting the drawn preform in such a manner as to prevent deposition of a silica cloud on the preform rods.
It is another object of the invention to provide an apparatus for melting an optical fiber preform to form preform rods, whereby deposition of a silica cloud on a tapered portion at opposite ends of preform rods can be effectively prevented.
According to one embodiment according to the present invention, there is provided a method for melting an optical fiber preform, which is obtained by drawing a large-sized mother ingot along a vertical direction, under heating conditions, and subsequently melting the resultant preform by use of a fusing burner to form preform rods having tapered portions at opposite ends thereof, wherein the preform is melted while blowing an oxygen gas from upper and lower sides relative to the fusing burner and whereby a silica cloud is prevented from deposition on the tapered portion of the preform rods.
According to another embodiment of the invention, there is also provided an apparatus for melting an optical fiber preform, which comprises a drawing unit having a rotary chuck, a feeding means, an electric furnace, and a drawing chuck, and fusing unit associated in connection with the drawing unit and having a fusing burner and a fusion chuck, wherein the fusing unit includes a plurality of nozzles located above and below the fusing burner unit and capable of blowing an oxidative gas against a preform being melted at an angle, xcex8, of blowing relative to the length of the preform, which is in the range of 20xc2x0#xcex8#60xc2x0.